As far as I can tell the iPhone multitouch framework sends the location of each touch but not the size.
I want to allow the user to tap a button with the tip of their thumb, or to put the pad of their thumb across multiple buttons (not real UIButtons) and have my touch tracking code realize it needs to press all the buttons the thumb is over.
Since UITouch handles the touch and nothing is said about the size, i'd say no you can't. You better try another approch like a drag..
I'm fairly sure that the hardware gets a very accurate read of the touched area, but the iPhone firmware does not bring this information to the developer. As CiNN said, UITouch just doesn't provide this information.
However, the hardware is the same as in the old Fingerworks.com products which really did provide the actual touch data to the developer. So perhaps in a (much) later OS release, touch surface profiles would be provided. it might be useful!